Power systems such as electrical power distribution or transmission systems are used to supply, transmit and use electric power. High Voltage Direct Current (HVDC) power transmission is becoming increasingly important due to increasing need for power supply or delivery and interconnected power transmission and distribution systems. Power systems such as electrical power distribution or transmission systems generally include a protection system for protecting, monitoring and controlling the operation and/or functionality of other components included in the power system, which other components hence may be referred to as protected units. Such protection systems may for example be able to detect short circuits, overcurrents and overvoltages in power lines, transformers and/or other parts or components of the power system. The protection systems can include protection equipment such as circuit breakers for isolating any possible faults for example occurring in power transmission and distribution lines by opening or tripping the circuit breakers. After the fault has been cleared, e.g. by performing repairs and/or maintenance on the component in which the fault has been detected, the power flow can be restored by closing the circuit breakers. In alternative or in addition the protection systems can be arranged to, upon detection of a fault in a particular route for power flow (e.g. in a certain component arranged in that route), isolate the route in which the fault has been detected and select an alternative route for the power flow, for example by means of a so called bypass switch which when closed may route the power flow so as to bypass the route in which the fault has been detected.
An HVDC converter station is a type of station configured to convert high voltage direct current (DC) to alternating current (AC) or the reverse. An HVDC converter station may comprise a plurality of elements such as the converter itself (or a plurality of converters connected in series or in parallel), an alternating current switch gear, transformers, capacitors, filters, a direct current switch gear and/or other auxiliary elements. Electronic converters may comprise a plurality of solid-state based devices such as semiconductor devices and may be categorized as line-commutated converters, using e.g. thyristors as switches, or voltage source converters, using transistors such as insulated gate bipolar transistors (IGBTs) as switches (or switching devices). A plurality of solid-state semiconductor devices such as thyristors or IGBTs may be connected together, for instance in series, to form a building block, or cell, of an HVDC converter, which may also be referred to as an HVDC converter valve.
According to one example, a plurality of solid-state semiconductor devices such as thyristors or IGBTs may be connected in series in a cell of an HVDC converter. During normal operation of e.g. an HVDC power transmission system or an HVDC grid including the HVDC converter, the solid-state semiconductor devices in the HVDC converter may at times be in a conducting mode in which they are conducting current and at other times be in a blocking mode, in order to attain a desired or required wave form of the current, as known in the art. This may expose the solid-state semiconductor devices to continuous current stresses, which especially in HVDC applications may be relatively high. If any one of the solid-state semiconductor devices would fail or malfunction, e.g. due to continuous current stresses, the current through the HVDC converter would be interrupted, and repairs and/or replacement of any failed solid-state semiconductor device might then become necessary in order to put the HVDC converter back into operation. In a HVDC converter station based on voltage source converters there may be DC capacitors, or DC capacitor banks, which act as voltage sources and which are connected to, for instance in parallel, one or several solid-state semiconductor devices such as IGBTs included in a cell of an HVDC converter.